Yes, ETH is required to pay gas fees (which is a couple cents).
Do you store my documents?
No, the contents of your document are never stored anywhere.
Are the contents of my documents safe?
Yes, only store the cryptographic hash of your documents is stored in the blockchain. The hash is calculated locally on your computer and serves as a fingerprint for your document. It is mathematically impossible to reconstruct your document from this hash.
docstamp.it is implemented using a smart contract on the Ethereum blockchain. When you record a document using docstamp.it, you are guaranteed that this record is immutable. This information cannot be lost, it cannot be modified even by governments and you can always verify this information even if docstamp.it the website is down.
Can I use docstamp.it for contracts or any signed documents?
Yes, you can scan a copy of the physically signed contract or other documents and stamp it using this service. When you need to timestamp data so that you can later prove that you possessed it.
When you need to preserve data integrity - The hash (Keccak-256) that is computed maintains a fingerprint of your document. This changes with any modification to the original document.
When you want the peace of mind that your document record is stored on a decentralized system that prevents any manipulation.
When you want to save yourself a trip to the notary.
No, all records of stamps will live on the blockchain forever.